Recently, code division multiple access (CDMA) technology has been proposed as one of promising radio transmission methods for implementing multimedia communications in the next generation mobile communication network systems. A CDMA system carries out diversity handover (DHO) that establishes multiple communication links (branches) between a mobile station and a plurality of base stations while the mobile station is moving near a boundary of a radio area, and communicates by performing signal combining (selection combining) on the multiple communication links. In the diversity condition, it is possible to increase a radio capacity by controlling such that the mobile station and the base stations can communicate at minimum transmission power (transmission power control). Furthermore, it is possible in the diversity condition to eliminate instantaneous chopping during handover, which can occur in the conventional time division multiple access (TDMA).
Establishing a new branch in the DHO is specifically referred to as “addition DHO” in the DHO, and deleting a communication branch not contributing to the communications in the diversity condition is specifically referred to as “deletion DHO” in the DHO. To carry out the addition DHO and deletion DHO, the mobile station normally detects a candidate of the addition DHO (called “addition DHO candidate” or “addition branch candidate”), and a candidate of the deletion DHO (called “deletion DHO candidate” or “deletion branch candidate”). Detecting an addition or deletion branch candidate by monitoring a radio condition (transmission loss, for example) of a perch in the current sector or peripheral sectors, the mobile station notifies the network side of the detected candidate. The network side performs the DHO (addition DHO or deletion DHO) based on the notified candidate. The term “network side” usually refers to a switching center or a control center of base stations, it is possible to provide this function to the base stations.
The mobile station communicates with the base stations using individual radio links (radio branches) associated with the base stations. A switching center comprises cable links (cable branches) connecting it with the base stations that are communicating with the mobile station, and carries out, with a handover (HO) processor or a diversity handover trunk (DHT), the selection combining of the signals sent from the base stations. The fundamental operation of the diversity handover branch is disclosed in Japanese Patent Application Laid-open No. 9-508773 (1997), and network configurations and control methods of the diversity handover trunks are disclosed in Japanese Patent Application No. 8-348900 (1996).
Because of hardware implementation or the like, there are provided an upper limit to the number of radio branches (a maximum radio branch number) that the mobile station can establish simultaneously, and an upper limit to the number of cable branches (a maximum cable branch number) that the DHT in the switching center can connect or process simultaneously. The upper limit to the number of communication branches (the maximum communication branch number) that can be established in the diversity condition equals the smaller one of the maximum numbers of the radio branches and cable branches. In ordinary systems, it is designed that the maximum radio branch number becomes equal to the maximum communication branch number to make effective use of radio resources by giving priority to them.
However, conventional papers (for example, Shimizu, et al. “Handover equipment and control method in next generation mobile communication systems”, General assembly of the Institute of Electronics, Information and Communication Engineers of Japan, 1997) handle the subject only under the assumption that the control of branches to be added or deleted is limited to a single branch, and do not handle simultaneous control of a plurality of addition branches and deletion branches. It is not specified in ITU-T recommendation Q.FIF version 6, as well.
As mentioned above, since the control of the branches to be added and deleted is carried out on one by one basis in the prior art, N times of control is required in principle to control N branches. However, the mobile station can detect a plurality of deletion branch candidates and addition branch candidates at the same time, because their detection depends on the ambient radio condition.
In such a case, a number of times of similar control operations are repeated between the mobile station and the switching center, which is not only inefficient, but also takes an extra time for the control until the entire handover control is completed.
Taking account of the addition DHO of a single branch, the prior art sets the maximum cable branch number at the maximum radio branch number plus one. Thus, the network side can prepare the branches in cable sections by the number greater than the maximum radio branch number by one. This enables a simple switching operation in the radio sections to complete the addition DHO by adding one cable branch from among the prepared cable branches without deleting the communication branch even in the case where one branch is added to the maximum number of branches.
However, such a method that follows the prior art, in which the maximum cable branch number is set at the number greater than the maximum radio branch number by one, has a problem of impairing effective control because of the network side control repeated by the number of addition branch candidates. Furthermore, no prior art takes account of handling any addition branch candidates that have not been prepared in the cable section, even though they have higher DHO priority than the communication branches, because the control unit is for a single branch and the maximum cable branch number is limited.